1. Field of the Invention
The invention relates to a torque detection device that detects, for example, a steering torque of a steering wheel and an electric power steering system that includes the torque detection device.
2. Discussion of Background
A torque sensor described in US 2004/0093961 A is formed of a torsion bar, a ring-shaped magnet, a pair of annular magnetic yokes, a magnetic sensor, and the like. The torsion bar couples an input shaft to an output shaft such that the input shaft and the output shaft are coaxial with each other. The magnet is attached to an end portion of the input shaft. The magnetic yokes are attached to an end portion of the output shaft. The magnetic sensor detects the density of magnetic fluxes induced by the magnetic yokes from the magnet. Each of the magnetic yokes has lugs that are arranged at equal intervals along the entire circumference of the magnetic yoke. The number of the lugs is equal to the number of N-poles and S-poles of the magnet. The magnetic sensor is placed in a gap formed between the axially facing magnetic yokes, and detects the magnetic flux density in the magnetic yokes.
When torque (steering torque) is input into a portion between the input shaft and the output shaft through a steering operation of a steering wheel, the torsion bar is twisted, and the relative position between the magnet and the magnetic yokes in the circumferential direction changes. The torque sensor detects the steering torque input into the portion between the input shaft and the output shaft on the basis of the magnetic flux density that changes in the magnetic yokes in accordance with the change in the relative position.
In the torque sensor described in US 2004/0093961 A, the shape of each lug of each magnetic yoke has an acute triangular or trapezoidal shape. In this case, the magnetic fluxes in each magnetic yoke are likely to unexpectedly leak from the acute-angled portion of each lug. Therefore, it is difficult for the magnetic sensor to highly accurately detect the magnetic flux density of the magnetic yokes. That is, it is difficult for the torque sensor to highly accurately detect a torque.